TD6/3: TIMBER

TIMBER

Timber refers to wood used for construction works. A tree that yields good wood for construction is called ‘Standing Timber.’ After felling a tree, its branches are cut and its stem is roughly converted into pieces of suitable length, so that it can be transported to timber yard. This form of timber is known as rough timber. By sawing, rough timber is converted into various commercial sizes like planks, battens, posts, beams etc. Such form of timber is known as converted timber.

Classification of Timber

Timber is mainly classified by:

• Mode of growth
• Modulus of elasticity
• Durability
• Grading

Classification Based on Mode of Growth:

On the basis of mode of growth trees are classified as;

• Exogenous
• Endogenous

a) Exogenous Trees:

These trees grow outward by adding distinct consecutive ring every year. These rings are known as annual rings. Hence it is possible to find the age of timber by counting these annual rings.

These trees may be further divided into;

• coniferous
• deciduous

b) Coniferous trees:

These are having cone shaped leaves and fruits. The leaves do not fall till new ones are grown. They yield soft wood.

Deciduous trees are having broad leaves. These leaves fall in autumn and new ones appear in springs. They yield strong wood and hence they are commonly used in building construction. The classification as soft wood and hard wood have commercial importance.

Difference between soft wood and hard wood.

1. In soft wood annual rings are seen distinctly whereas in hard wood they are
2. The colour of soft wood is light whereas the colour of hard wood is
3. Soft woods have lesser strength in compression and shear compared to hard
4. Soft woods are light and hard woods are
5. Fire resistance of soft wood is poor compared to that of hard
6. The structure of soft wood is resinous while structure of hard wood is close grained

The cross-section of an exogenous tree is as shown.

1. Pith: it is the inner most part of the tree and hence the oldest of exogenous tree when the plant becomes old, the pith dies and becomes fibrous and dark. It varies in size and
2. Heart Wood: This is the portion surrounding pith. It is dark in colour and strong. This portion is useful for various engineering purpose. This is the dead part of wood. It consists of several annular rings.
3. Sap Wood: It is the layer next to heart wood. It denotes recent growth and contains sap. It

takes active part in the growth of trees by allowing sap to move in upward direction. The annual rings of sap wood are less sharply divided and are light in colour. The sap wood is also known as alburnum.

4. Cambium Layer: It is a thin layer of fresh sap lying between sap wood and the inner bark. It contains sap which is not yet converted into sap wood. If the bark is removed and cambium layer is exposed to atmosphere, cells cease to be active and tree
5. Inner Bark: It is an inner skin of tree protecting the cambium layer. It gives protection to cambium
6. Outer Bark: It is the outer skin of the tree and consists of wood fibres. Sometimes it contains fissures and
7. Medullary Rags: These are thin radial fibres extending from pith to cambium layer. They

hold annular rings together. In some of trees they are broken and some other they may not be prominent.

b) Endogenous Trees:

These trees grow inwards. Fresh fibrous mass is in the inner most

portion. Examples of endogenous trees are bamboo and cane. They are not useful for structural works.

Properties of Timber

Properties of good timbers are:

1. Colour: It should be
2. Odour: It should be pleasant when cut
3. Soundness: A clear ringing sound when struck indicates the timber is
4. Texture: Texture of good timber is fine and even.
5. Grains: In good timber grains are close.
6. Density: Higher the density stronger is the
7. Hardness: Harder timbers are strong and
8. Warping: Good timber do not warp under changing environmental
9. Toughness: Timber should be capable of resisting shock
10. Abrasion: Good timber do not deteriorate due to wear. This property should be looked into, if timber is to be used for
11. Strength: Timber should have high strength in bending, shear and direct
12. Modulus of Elasticity: Timber with higher modulus of elasticity are preferred in
13. Fire resistance: A good timber should have high resistance to
14. Permeability: Good timber has low water
15. Workability: Timber should be easily workable. It should not clog the
16. Durability: Good timber is one which is capable of resisting the action of fungi and insects attack
17. Defects: Good timber is free from defects like dead knots, shakes and

Seasoning of Timber

This is a process by which moisture content in a freshly cut tree is reduced to a suitable level. By doing so the durability of timber is increased. The various methods of seasoning used may be classified into:

• Natural seasoning
• Artificial seasoning.

Natural Seasoning: It may be air seasoning or water seasoning.

1. Air seasoning is carried out in a shed with a platform. On about 300 mm high platform timber balks are

Care is taken to see that there is proper air circulation around each timber balk. Over a period, in a natural process moisture content reduces. A well-seasoned timber contains only 15% moisture. This is a slow but a good process of seasoning.

1. Water seasoning is carried out on the banks of rivers. The thicker end of the timber is kept pointing upstream side. After a period of 2 to 4 weeks the timber is taken out. During this period sap contained in the timber is washed out to a great extent. Then timber is stalked in a shed with free air circulation.

Artificial Seasoning: In this method timber is seasoned in a chamber with regulated heat, controlled humidity and proper air circulation. Seasoning can be completed in 4 to 5 days only. The different methods of seasoning are:

• Boiling
• Kiln seasoning
• Chemical seasoning
• Electrical seasoning.
  • Boiling: In this method timber is immersed in water and then water is boiled for 3 to 4 hours. Then it is dried slowly. Instead of boiling water hot steam may be circulated on timber. The process of seasoning is fast, but
  • Kiln Seasoning: Kiln is an airtight chamber. Timber to be seasoned is placed inside it. Then fully saturated air with a temperature 35°C to 38°C is forced in the kiln. The heat gradually reaches inside timber. Then relative humidity is gradually reduced and temperature is increased, and maintained till desired degree of moisture content is

The kiln used may be stationary or progressive. In progressive kiln the carriages carrying timber travel from one end of kiln to other end gradually. The hot air is supplied from the discharging end so that temperature increase is gradual from charging end to discharging end. This method is used for seasoning on a larger scale.

• Chemical Seasoning: In this method, the timber is immersed in a solution of suitableThen the timber is dried in a kiln. The preliminary treatment by chemical seasoning ensures uniform seasoning of outer and inner parts of timber.
  • Electrical Seasoning: In this method high frequency alternate electric current is passed through timber. Resistance to electric current is low when moisture content in timber is high. As moisture content reduces the resistance reduces. Measure of resistance can be used to stop seasoning at appropriate

  However, it is costly process. This technique has been tried in some plywood industries but not in seasoning of timber on mass scale.

  Defects in Timber

  Various defects which are likely to occur in timber may be grouped into the following three:

  • Due to natural forces
  • Due to defective seasoning and
  • Due to attack by fungi and

  Defects due to Natural Forces: The following defects are caused by natural forces:

  (a) Knots (b) Shakes

  (c) Wind cracks (d) Upsets

  • Knots: When a tree grows, many of its branches fall and the stump of these branches in the

  trunk is covered. In the sawn pieces of timber, the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion.

  If the knot is intact with surrounding wood; it is called live knot. If it is not held firmly it is dead knot.

• Shakes: The shakes are cracks in the timber which appear due to excessive heat, frost or twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes

• Wind Cracks: These are the cracks on the outside of a log due to the shrinkage of the exterior surface. They appear as

• Upsets: Figure shows a typical upset in a timber. This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.

• Defects due to Defective Seasoning and Conversion:

If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. These are more susceptible in case of kiln seasoning.

In the process of converting timber to commercial sizes and shapes the following types of defects are likely to arise: chip marks, torn grain etc.

• Defects due to Fungi and Insects Attack:

Fungi grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.

Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.

Preservation of Timber

Preservation of timber means protecting timber from fungi and insects attack so that its life is increased. Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:

1. Tar
2. Paints
3. Chemical salt
4. Creosote
5. ASCO

Tar: Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.

Paints: Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. So lignum paint is a special paint which protects the timber from the attack of termites.

Chemical salt: These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride.

After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.

Creosote: Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in

an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a controlled pressure. After 1 to 2 hours’ timber is taken out of the chamber.

ASCO: This preservative is developed by the Forest Research Institute, Dehradun. This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber.

This treatment prevents attack from termites. The surface may be painted to get desired appearance.

Uses of Timber

Timber is used for the following works:

1. For heavy construction works like columns, trusses,
2. For light construction works like doors, windows, flooring and
3. For other permanent works like for railway sleepers, fencing poles, electric poles and
4. For temporary works in construction like scaffolding, centering, shoring and strutting, packing of
5. For decorative works like showcases and
6. For body works of buses, lorries, trains and boats
7. For industrial uses like pulps (used in making papers), card boards, wall papers
8. For making sports goods and musical instruments